You can change the order of your accounts, and your nominated favourite account on the Accounts page.
Once on the Accounts page, tap the bolt icon on the right of the account name that you would like to make your favourite account.
You can also tap and hold the account to change the order. When you are happy with the position simply release from holding.
The account that you leave at the top of this list is your default account and will be the first on the list whenever you are selecting an account to transfer from.